Passenger aircraft fly at high altitudes in order to conserve fuel and achieve an adequate flight range. For example, large passenger aircraft typically cruise at a height of between 7500 meters and 14000 meters, whilst Concorde can cruise at 18000 meters or more.
Although the proportion of oxygen in the atmosphere remains constant irrespective of altitude, i.e. the air composition remains at 21% oxygen, 79% nitrogen, as altitude increases the atmospheric pressure decreases, as illustrated in Table 1.
TABLE 1Ambient Pressure at Different AltitudesAltitudeAltitudeAmbient PressureAmbient PressureFeetMetresKpaPsi  00101.414.7 4750144884.012.19 8000243876.710.8310000304872.010.4412000365864.09.2815000457257.48.3517500533450.77.3525000762038.65.6360001097322.73.29400001219218.72.71440001341116.02.3260000182887.31.06
A decrease in atmospheric pressure is accompanied by a reduction in the ability of humans to transfer oxygen from the lungs to the bloodstream. As altitude increases, the atmospheric pressure falls and humans become much more vulnerable to the effects of hypoxia.
To protect passengers and crew from hypoxia, passenger aircraft flying above 3000 meters have to be pressurised to compensate for the reduced atmospheric pressure.
To allow aircraft to be pressurised, the fuselage of aircraft has to be strengthened, so that the fuselage can act as a pressure vessel able to contain an internal pressure much higher than the external atmospheric pressure. Strengthening the fuselage increases the cost of an aircraft, and the increased weight of the fuselage also has a significant impact on the fuel consumption and potential range of the aircraft.
In order to reduce the amount of strengthening required to the aircraft fuselage, the cabin pressure is usually maintained at about 76.7 kPa, i.e. 10.83 psi. This pressure is equivalent to the ambient pressure at an altitude of 8000 feet, i.e. 2,500 meters, rather than the ambient pressure of 101.4 kPa that prevails at sea level. Pressurising the aircraft to only 76.7 kPa saves weight and reduces operating costs.
However, the partial pressure of the oxygen inside an aircraft pressurised to 76.7 kPa is about 25% less than the corresponding pressure at sea level, and prolonged exposure to this lower partial pressure could still have an adverse affect on the health of passengers. The cabin pressure in commercial passenger aircraft is therefore a compromise between the degree of hypoxia that passengers and crew are expected to be able to tolerate, and the strength and weight of the aircraft.
Decisions regarding the optimum cabin pressure to be used inside passenger aircraft were made at a time when passenger numbers were much lower than today and also when far fewer passengers participated in long-haul flights. It was probably also assumed at the time that most airline passengers were fit and healthy, whereas many passengers travelling today are elderly, some are obese and some suffer from ailments that make them much more vulnerable to hypoxia.
For example, passengers particularly at risk of hypoxia include those with pulmonary and heart diseases who find breathing difficult and have impaired oxygen uptake even under normal atmospheric conditions. Their breathing difficulties would be exacerbated under reduced oxygen partial pressure conditions, and hypoxia in such vulnerable passengers could well precipitate heart attacks and strokes during flight.
There is also an increased risk of clots forming in the deep veins of legs, particularly during long-haul flights because of the combination of hypoxia and long periods of immobility inside aircraft cabins, and such deep vein clots could lead to pulmonary embolism after landing.
The incidence of passengers suffering from hypoxia related ailments does appear to be on the increase. Worryingly, cases have also been reported where fit, younger, passengers have apparently suffered from hypoxia related illnesses either during or after long-haul flights.
The number of flights to long-haul destinations has increased dramatically over recent years and many more passengers are now being subjected to reduced atmospheric pressure conditions and inactivity for longer periods of time. The apparent spread of hypoxia related ailments to younger people also suggests that this combination of poor atmospheric conditions and immobility may well be of concern to all passengers, irrespective of their age, health and general fitness.
Modern passenger aircraft are equipped with sophisticated air conditioning systems. The air conditioning systems have primarily been designed to prevent an excessive build up of carbon dioxide in the breathable air inside passenger cabins, whilst also providing a comfortable in-flight environment for passengers.
Carbon dioxide in the atmosphere inside aircraft can come from the vapourisation of dry ice, which is used to chill food in the galleys, as well as from human respiration. Carbon dioxide is an asphyxiant, and as little as a 2% concentration of carbon dioxide in the atmosphere can initiate headaches, increase blood pressure and cause deeper respiration. The air transport regulatory authorities specify that 3% is the maximum amount of carbon dioxide allowed in the breathable air inside passenger aircraft, although 3% carbon dioxide is known to produce weak narcosis in some people.
Understandably, from a passenger health and safety point of view, the prevention of excessive build-up of carbon dioxide has always been regarded as the most important requirement of an aircraft air conditioning system. The amount of carbon dioxide in the atmosphere inside the passenger cabins is controlled by regularly replenishing the air inside the aircraft with fresh compressed air supplied by the engines.
However, there are cost implications in taking compressed air from the engines for use other than for motive power. For example, a typical aircraft air conditioning system could well account for between 3% and 4% of the total aircraft fuel consumption. To help control these energy costs, about 40% of the stale air extracted from passenger cabins is recirculated and reused inside the aircraft.
The stale air is filtered, to remove airborne aerosols and dust, and the filtered air is then mixed with fresh compressed air from the engines before being recirculated back into the passenger cabins. The recirculated air will, however, contain less oxygen and more carbon dioxide than the fresh compressed air supplied by the engines, because the recirculated air will already have been used for respiratory purposes.
An important benefit associated with the reuse of air from the passenger cabins is that the stale air contains moisture from the respiration of passengers, whereas the fresh compressed air from the engines tends to be extremely dry. The recirculated air therefore provides the humidity needed to prevent the dehydration of passengers.
Maintaining passenger comfort is also an important feature of an aircraft conditioning system. For example, changes in cabin pressure during take-off and landing are carefully controlled by the air supply system, to ensure that the ear drums of passengers are not subjected to sudden or excessive stress. During flight, the air inside the passenger cabins is maintained at temperature, humidity, pressure and flow rate conditions that provide a comfortable as well as a safe environment to passengers.
Until recently, however, very little attention has been given to the concentration of oxygen in the breathable atmosphere supplied to passengers inside aircraft. For example, an alternative to pressurising passenger aircraft, in order to provide an acceptable partial pressure of oxygen, would be to increase the amount of oxygen in the breathable air supplied to the passengers.
This is in fact the approach used for military aircraft, where it is common practice for the pilot to have an individual supply of oxygen to compensate for the reduced partial pressure of oxygen at high attitudes. From a military point of view, using oxygen to improve breathing has the advantage that the military aircraft can be kept as light as possible because they are not pressurised to the same degree as passenger aircraft.
However, modern passenger aircraft carry large numbers of passengers, and it is not practical to use pure oxygen to increase the oxygen concentration in the breathable atmosphere inside the aircraft. For example, packaged oxygen, i.e. oxygen in the form of either compressed gas in cylinders or liquid in tanks, would not only be extremely expensive, but would also have serious weight implications for the aircraft.
Oxygen production systems, such as pressure swing adsorption, vacuum swing adsorption and cryogenic systems, are commercially available to produce either pure or high purity oxygen for industrial processes.
However, industrial oxygen production systems are very large, heavy and energy intensive, and such systems are not a practical solution for passenger aircraft applications.
There are also significant safety implications associated with the storage and use of pure oxygen. For example, pure oxygen encourages spontaneous combustion, and care has to be taken at all times to ensure that pure oxygen is always isolated and kept well away from fuels and other combustible materials.
The alternative to using pure oxygen would be to use an oxygen concentrator, which uses gas separation membranes to separate normal atmospheric air into an oxygen rich fraction and a nitrogen rich fraction.
An earlier European patent application by the applicant, EP 0808769 A, filed on the 20th May 1997, described how, in theory, oxygen concentrators could be used to supply enriched oxygen air to the passenger cabins of aircraft. However, very high volumes of air are required to conform with the passenger air supply regulations specified by the air transport authorities, and the industrial oxygen concentrators available at the time of the above patent application were unable to efficiently supply such large amounts of enriched oxygen air.
For example, the air transport regulatory authorities specify that each passenger should be supplied with about 10 ft3/min of fresh air. On large passenger aircraft, which can carry up to 550 passengers plus crew, this amount of fresh air per passenger would be equivalent to a total air supply of some 6000 ft3/min, i.e. about 165 m3/minute.
Industrial oxygen concentrators generally have relatively low gas flux and they usually require high operating pressures, of at least 7-bar pressure, to force the air through the gas separation membranes inside the concentrator. The need to use high pressure to produce the oxygen rich air imposes high energy demands on the gas separation system, and their low gas flux also limits the volume of oxygen rich air that can be produced from such concentrators.
Because of their high pressure operation, industrial oxygen concentrators have to be made from heavy, pressure resistant materials. It is therefore impractical to use industrial oxygen concentrators to produce the large amounts of oxygen rich air needed to meet the requirements of a typical passenger aircraft air conditioning system.